The present invention relates to a valve mechanism for an automotive engine and more particularly to the valve mechanism where a valve lift and a valve timing are varied in accordance with engine speed.
In an automotive engine, a valve opening angle which is a cam angle between the valve opening and closing, and the valve lift of the valves have a large influence on volumetric efficiency of the engine, stability of low engine speed and high engine speed performance. Namely, it is preferable to set the valve opening angle and the valve lift at small values respectively in a low and middle engine speed ranges, and to set the opening angle and the lift at a large value in a high engine speed range.
In order to change the valve opening angle and the valve lift, there has been proposed a valve mechanism where a low-speed cam for the low and middle engine speed range, a high-speed ca for the high engine speed range and rocker arms corresponding to the respective cams are provided. One of the rocker arms is operatively connected to the valve and both rocker arms are connected or disconnected from each other in accordance with the engine speed so as to select a necessary cam. Hence the valves are operated to open at a small lift and a small opening degree at a low engine speed and to open at a large lift and a large opening degree at a high engine speed.
U.S. Pat. No. 5,020,488 discloses such a valve mechanism where both rocker arms are mechanically connected. A pair of low-speed cams and a high-speed cam are formed on a cam shaft. A pair of roller followers are rotatably mounted on a connecting shaft provided to connect both the low-speed rocker arms. A part of a peripheral portion of roller follower is projected from the rocker arm to be engaged with the low-speed cam. The high-speed rocker arm has a slipper engaged with the high-speed cam. A changeover pin is slidably mounted in the high-speed rocker arm to be engaged with the connecting shaft so as to connect the high-speed rocker arm with the low-speed rocker arms.
The connecting shaft has an annular groove having a semicircular cross section at the periphery thereof. The radius of the semicircle is set to a larger value than the radius of the changeover pin so that the pin can be smoothly engaged with the groove. However, such a larger diameter causes a point contact between the wall of the groove and the changeover pin. As a result, contact pressure at the contact point increases, which causes wear and tear of the parts.
On the other hand, the changeover pin is formed with a piston and a rod and is slidably mounted in a piston chamber formed in the high speed rocker arm. The pin is projected by oil pressure applied to an oil chamber and retracted by a spring provided in a spring chamber. The rod has a small diameter so as to form the piston and to provide the spring. Consequently, the changeover pin has a low stiffness and low mechinability. The oil chamber and the spring chamber which are separated by a partition make the construction thereof complicated and occupy a large axial space, which makes the high speed rocker arm large.